Not applicable.
Not applicable.
1. Field of the Invention
The preferred embodiments of the present invention are generally directed to reducing crosstalk and other noise sources in downhole tools. More particularly, the preferred embodiments are directed to implementing a phase-reversal element that enables an electromagnetic resistivity tool""s circuitry to quantify and/or nullify crosstalk and other noise sources.
2. Background of the Invention
In the exploration and production of hydrocarbons in an underground reservoir or formation, it is often desirable to have downhole parameters and information readily available. For example, when exploring a formation by drilling, it is desirable to know formation properties in order to determine if a reservoir has been encountered. Methods and apparatuses for determining and measuring downhole parameters are well known in the art, and may include generating an electromagnetic stimulus using transmit circuitry that penetrates the formation while the formation""s response to the electromagnetic stimulus is acquired with receive circuitry.
Due to confined downhole conditions, the electromagnetic transmit and receive circuitry are often located in close proximity to each other. Additionally, because of formation attenuation, the receive circuitry operates at a lower power level (e.g. several orders of magnitude) than that of the transmit circuitry, causing almost any electromagnetic interference (EMI) generated by the transmit circuitry to affect the operation of the receive circuitry drastically. This contamination of EMI between the transmit and receive circuitry leads to a condition commonly know as crosstalk.
Current trends in attempting to solve crosstalk and other noise related problems in downhole applications include separating the transmit and receive circuitry physically, but this may lead to, among other things, redundant power circuitry. Filtering circuitry that the transmit/receive circuits share (e.g. power supplies, reference oscillators, etc.) has also been attempted, but it does not provide the desired performance. Shielding the affected circuit may also be employed in effort to reduce crosstalk, however, this is often relatively expensive and does not provide adequate performance.
Thus, there is a need for an apparatus and method to reduce crosstalk in downhole tools that is both cost effective and also provides improvements in the desired level of performance.
The problems noted above are solved in large part by a phase-reversal element employed so that the phase of the signal coming from a receive sensor, which represents the formation""s response to the electromagnetic stimulus, may be reversed prior to being processed by the receive circuitry. As such, the phase-reversal element is preferably located in close physical proximity to the sensor so that the maximum amount of crosstalk between the transmit circuitry and the receive circuitry is reduced by subsequent digital signal processing circuitry. In the preferred embodiments, the signals processed by subsequent circuitry are a composition of the original signal measured by the sensor, followed by a phase-reversed version of the same signal. The receive circuitry then applies the remainder of the analog and digital signal processing to the resultant signal, where the crosstalk may be both quantified and nullified digitally.
In another embodiment, a phase-reversal element may be employed at any point in the receive path circuitry. In this manner, the subsequent digital signal processing circuitry may quantify and nullify any crosstalk or noise that comes after the phase-reversal element. For example, if it is known that the signal conditioning circuitry is the most susceptible to crosstalk, then the phase-reversal element may be employed just before the signal conditioning circuitry. Therefore, if it is undesirable to implement the phase-reversal element in close proximity to the sensor, placing it proximate to elements that are known to be more susceptible than others may be an alternative embodiment.
The disclosed devices and methods comprise a combination of features and advantages which enable it to overcome the deficiencies of the prior art devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings.